The invention relates to video displays for a vector analyzer. One aspect of the invention is a set of markers for the display of a vector analyzer. Another aspect of the invention is a circuit for performing transforms on data which allows real time adjustment of the sampled signals and enables the signals to be displayed in a three dimensional projection mode.
A vector analyzer is a test instrument for use with vector modulated (I/Q modulated) signals. Vector modulation is becoming widespread in radar, microwave and satellite communications and other pulsed component systems. In vector modulation, a carrier signal is modulated by two modulation signals in quadrature: the in phase I, and the quadrature Q modulation signals. A modulated signal can be demodulated with a quadrature detector. The quadrature detector downconverts the modulated signal with two coherent reference signals 90 degrees apart. One downconversion recovers the I component and the other recovers the Q component. For a general discussion of vector modulation and visual displays, see, "Program Helps Teach Digital Microwave Fundamentals", Hewlett-Packard Journal, April 1986, pp. 40-46.
Several vector modulation schemes have evolved, including BPSK, QPSK, 8PSK, AND 16QAM. The resulting modulation patterns can be plotted on a special X-Y display with the I amplitude on the X axis and the Q amplitude on the Y axis.
A vector analyzer measures the amplitude and phase of modulated microwave and RF signals by characterizing the I and Q outputs of quadrature detectors. Because of the complexity of the modulation patterns, visual analysis is important. The vector analyzer has four basic modes of displaying the I and Q signals with respect to each other and with respect to time.
The state or vector diagram mode displays the I component vs. the Q components for a time interval in an X-Y display. The constellation mode displays I vs. Q at a time instant. The EYE diagram mode displays the I or Q component vs. time. The fourth mode is a three dimensional projection of all three variables, I, Q and time onto the X-Y plane.
FIG. 2 shows examples of the four modes for a QPSK modulated signal. FIG. 2a shows the vector mode, FIG. 2b shows the constellation mode, FIGS. 2c and 2d show the I and Q EYE diagram modes respectively, and FIG. 2e shows the three dimensional mode.
One purpose of the vector analyzer is to find any errors in the phase or magnitude of the modulation states displayed. Visual analysis of errors can be very difficult, especially for complex modulation schemes such as 16QAM.
Without display markers for phase and magnitude of the modulation states, the operator would have to calculate the phase and magnitude values and errors from the I and Q values of the states using rectangular-to-polar coordinate conversion formulas.
Other signal analysis instruments have used display markers to aid the visual analysis of the signals. One example is a spectrum analyzer with amplitude and frequency markers, described in U.S. Pat. No. 4,257,104, "Apparatus for Spectrum Analysis of an Electrical Signal", issued Mar. 17, 1981, and in U.S. Pat. No. 4,253,152, "Spectrum Analyzer with Parameter Updating from a Display Marker", issued Feb. 24 1981. However, these markers operated only in the cartesian coordinates of the spectrum analyzer display.
An object of the invention is to provide a set of display markers to aid the analysis of the modulation states displayed by the various display modes of a vector analyzer, operating in both cartesian and polar coordinates.
The vector analyzer of the invention incorporates a set of five types of markers for the display, which facilitate the visual analysis of the magnitude, phase and time relationships of the I and Q components of the modulation states displayed. The five markers are: an I marker, a Q marker, a magnitude marker, a phase marker and a time marker. The I and Q markers indicate amplitude in the I and Q coordinates displayed on the vector and constellation X-Y displays or on the I, Q, and I&Q displays. The magnitude marker indicates amplitude on the radial coordinate of the vector and constellation X-Y displays. The phase marker indicates relative phase on the angular coordinate of the vector and constellation X-Y displays. The time marker indicates the time coordinate on the I, Q, and I&Q displays, and to control the time instant displayed on the constellation display.
In addition to the markers, another aspect of the invention is a circuit for performing linear transforms on the I, Q, and time variable data which allows real time adjustment of the sampled signals and enables the signals to be displayed in a three dimensional projection mode.
This linear transform may be used to correct the display for three common sources of error in vector demodulators: amplitude imbalance, phase errors, and quadrature errors. After the display has been corrected, the correction factors are a measure of the errors in the device demodulating the input signal, or in the input signal itself.
In the three dimensional projection mode the I, Q and time coordinates can be visually compared. At any point in time there is a relationship between these three variables, but the customary oscilloscope displays show only two of the three variables.